1. Filed of the Invention
The present invention is concerned with head-up displays. Such displays allow an exterior view and a collimated symbolic image to be visualized simultaneously.
The invention can be applied whenever a symbolic image is to be positioned precisely with respect to the exterior view. This is the case whenever the symbolic image represents an element of the exterior view, for example, in an aircraft, when the symbolic element might be a landing strip or, in a military aircraft, when the symbol might represent a target.
2. Description of the Prior Art
Head-up displays of this kind, in general, incorporate a means of generating a symbolic image, an optical unit for projecting the image, and a combiner that consists of a partially transparent plate on which the projected image is received before being sent on towards the eye of an observer such as a pilot.
The problem of correctly positioning the symbolic image is well known in the prior art. It seems, however, that the need to reduce any positioning error to within a maximum value beyond which "flight by sight" in bad visibility would not be possible, has not been fully appreciated. Ways of detecting and correcting the errors due to bad positioning of the combiner have been described, for example, in patent U.S. Pat. No. 4,775,218.
In the device described in this patent, a light source, fixed to the structure of an aircraft, illuminates a mirror which is attached to a combiner and which reflects the light from the source towards a linear position sensor. This arrangement of sensor and light source can be used to detect a bad angular position of the combiner, measured about the axis about which the combiner can be turned up and down.
The signal generated by the sensor is processed by signal processing circuits and used to produce vertical movement of the image generated by a cathode ray tube, in order to compensate for the error in the position of the image due to bad positioning of the combiner.
The device described in the above mentioned patent has three drawbacks:
There is no correlation between the device and the means of projection; the device does not control the position of the combiner with respect to the image generation system. PA1 The position is controlled along one axis only (line of site). PA1 The absolute position of a symbol in the field of view, with respect to a reference point, is not guaranteed. PA1 1) The device described in the patent is not intended as a device for controlling the position of the image, but as a device for controlling the relative position of the combiner with respect to a nominal position. The light source and the sensor are positioned with respect to the structure of the support and not with respect to the image generating device. PA1 2) The device controls the position along a single axis only, and is intended only to compensate bad angular positioning of the combiner about an axis of rotation contained in or parallel to the plane of the combiner. PA1 3) Good positioning of the combiner does not guarantee a good absolute position of the symbolic image since the positioning error of the combiner is only one of a number of sources of error. PA1 Electronic errors due to tube drift errors: deflection, very high voltage, offset and gain in X-Y circuits, all of which could cause a shift in the position of the image with respect to a nominal position. PA1 Opto-mechanical errors due to bad angular positioning of the combiner with respect to the relay optics, which can be caused by mechanical wear that may occur, during normal use of the combiner. PA1 The global error, measured by testing the entire image positioning system. PA1 The opto-mechanical error, measured by testing the angular position of the combiner. PA1 a means of generating a luminous spot at a known position on a geometric surface on which the symbolic image is to be found, PA1 an optical focusing system, PA1 a first sensor capable of detecting position, incorporating a light sensitive surface that generates a signal that depends on the position of an incident light beam, PA1 a mirror fixed directly to the combiner or to a structural component attached in turn to the combiner, and PA1 a first means of processing the signals output by the sensor, PA1 the light beam emitted by the luminous spot being collimated by the symbolic image collimating unit and reflected by the mirror through the focusing system towards the first position sensor. PA1 It is known that the difference between the signals of two sectors of the sensor constitute a signal whose amplitude varies according to variations in the incident light they each receive. This difference is assessed for each pair of sectors by comparing the measured difference value to reference difference values which, in the case of the invention, correspond to a good position. These difference signals are amplified, processed, and used to control, in a predictable manner, the values of the voltages that determine the position of the symbolic image.
These three points are developed in the following paragraphs: